This invention relates generally to apparatus for forming multi-color electrographic images, and more particularly, to apparatus for accurately forming color-corrected electrographic images.
Electrographic technology commonly employed in making monochrome reproductions has recently been adapted to the making of multi-color reproductions. Examples of reproduction apparatus using such technology in the making of multi-color reproductions is shown in U.S. Pat. Nos. 3,841,751, issued Oct. 15, 1974 in the name of Draugelis et al., and 4,436,405, issued Mar. 14, 1984 in the name of Kindt. In such apparatus, discrete areas of a uniformly charged dielectric member (or discrete dielectric members) are electrographically processed to alter such charge to form charge patterns corresponding respectively to color separation images (e.g., red, green, blue) of multi-color information to be reproduced. The charge patterns are then developed respectively with complementary colored electroscopic marking particles (e.g., cyan, magenta, yellow) and successively transferred in accurate superimposed register to a receiver sheet to form the multi-color reproduction.
One problem encountered in obtaining faithful multi-color reproductions using electrographic apparatus is related to light absorption characteristics of the electroscopic marking particles. For example, cyan pigments or dyes used in such particles often exhibit unwanted green and blue light absorption in addition to desired red light absorption. Similarly, magenta pigments or dyes used in such particles often exhibit unwanted blue light absorption in addition to desired green light absorption. If the unwanted light absorptions are not accounted for (corrected), the produced multi-color reproduction can be degraded in the fidelity of its color saturation and hue.
A related problem is that of exposure error of the red, green and blue information in the multi-color information being reproduced. There are usually side absorptions in the dyes, inks or toners used in such information. That is, when the charge patterns are formed by exposure through a color-separation filter system, and such system is not precisely matched for a particular input colorant set, the amount of cyan, magenta and yellow marking particles used in the reproduction will not be precisely proportional to the amount of cyan, magenta and yellow colorant in the information being reproduced. For example, the amount of yellow particles used in the reproduction will include an amount in proportion to the amount of yellow colorant in the information being reproduced plus amounts in proportion to the amounts of cyan and magenta colorant in such information weighted by their respective blue absorptions within the passband of the blue color-separation filter which is used. Imperfect matching of the blue filter to the input colorants can cause the amount of yellow marking particles used to differ from the amounts of such input colorants in the information being reproduced, thereby degrading the saturation and hue fidelity of the reproduction relative to such information.
A variety of solutions have been suggested for "color correction" of unwanted light absorptions of the marking particle colorants. For example, U.S. Pat. Nos. 3,615,391; 3,836,244 and 3,844,783 disclose color correction techniques wherein an element bearing an electrostatic mask pattern is placed into facing relation with a photoconductor sector which bears an electrostatic color-separation image. Development then occurs with the two electrostatic patterns competing for marking particles. Alternatively, an approach similar to graphics arts masking can be used. This involves forming a negative masking marking particle image and exposing the electrostatic color-separation image to the information being reproduced through the masking image. These techniques involve additional steps, are difficult to control accurately, and are difficult to implement in an automated machine.
U.S. Pat. No. 4,236,809 suggests performing color correction of a color-separation electrostatic image by selectively discharging it with a scanning laser beam (controlled in accordance with an electrical signal obtained from a previous electro-optic scan of the information being reproduced). U.S. Pat. No. 4,090,876 discloses a device using first and second ion modulating screens to form and color correct electrostatic color-separation images. Both of these latter approaches involve complex and expensive equipment additions to the electrographic apparatus, with the inevitably coupled problems in maintenance and reliability.
A much simplified solution to color correction is described in the commonly assigned U.S. patent application Ser. No. 493,867, filed May 12, 1983, in the name of Spitzner et al. According to the disclosure of such application, a plurality of photoconductor sectors are exposed to multi-color information to be reproduced through a plurality of primary color filters respectively to form electrostatic charge patterns corresponding to color separation images of such information. A charge pattern of a first color-separation image is developed with pigmented marking particles of one color (e.g., cyan). A reflected light image of such developed charge pattern exposes, in register, the charge pattern of another color-separation image (e.g., pattern formed with a green filter) prior to development with its respective pigmented marking particles of another color (e.g., magenta). Such exposures provides the desired color correction by altering the subsequent charge pattern in a manner which proportionally reduces the amount of marking particles utilized in developing such pattern by a degree which substantially compensates for the unwanted light absorption characteristics of the marking particles of the previously developed charge pattern. The accuracy of registration between the developed image and a subsequent charge pattern during color correction exposure is essential in obtaining the desired alteration of such charge pattern. In the embodiments of the Spitzner et al application where discrete photoconductor sheets are utilized, the sheets are shuttled along complex paths and precise registration for color correction is not necessarily ensured.